1) Field of the Invention
The present invention generally relates to a vehicular four wheel drive assembly and to a method for utilizing the same, and more particularly to a vehicular four wheel drive assembly having at least two modes of operation, the first operational mode being effective to sense slip and to selectively transfer torque to at least one axle in response to the sensed slip and the second operational mode also being effective to transfer torque to the at least one axle in response to the occurrence of a certain condition which may increase the likelihood of slip, the second operational mode occurring only after slip has actually occurred or has been detected.
2) Background of the Invention
A four wheel drive assembly typically includes a torque transfer assembly which receives torque from a rotating crankshaft and which transfers at least a portion of the received torque to a first and/or a second driveshaft which are respectively coupled to the front and rear axles. More particularly, the four wheel drive assembly includes a controller which is typically operated under stored program control and one or more sensors which are physically and communicatively coupled to the controller. The controller is operatively coupled to the torque transfer assembly and, upon the occurrence of one or more sensed events or conditions, causes the torque transfer assembly to transfer torque to the front and/or rear axle.
Particularly, should one of the axles (or the wheels which are mounted upon the one axle) rotate at a speed which is greater than the speed of a second of the axles or the wheels which are mounted upon the second axle (i.e., often referred to as a slip condition), by a certain threshold amount, the controller causes the torque transfer assembly to increase the amount of torque being transferred to one of the moving axles or wheels, such as and without limitation the slower moving axle, and reduce the amount of torque being transferred to the other moving axle or wheels, thereby slowing the speed of the other (e.g., the faster) moving axle and wheels.
A second operating strategy (e.g., a “preemptive” strategy) causes such torque reallocation to occur when certain conditions or events are sensed, in the absence of actual slip. For example, whenever the difference in axle or wheel speeds exceed a certain value and the accelerator member and/or the throttle plate are positioned in a certain manner, torque allocation is achieved. In this manner, the likelihood of the occurrence of slip is reduced.
While the second operating strategy does desirably decrease the likelihood of slip from occuring, it suffers from some drawbacks. For example and without limitation, the second strategy is employed whenever the vehicle is activated or “running” and the provided torque allocation is dependent only upon vehicular speed and the position of one or more certain components. As such, this strategy is often utilized even when there is very little likelihood of the occurrence of slip, such as when the vehicle is driven upon a surface which maintains a relatively large frictional contact with the wheels of the vehicle (e.g., surfaces having a relatively high coefficient of friction or “μ”). The unnecessary employment of the “torque allocation” strategy unnecessarily increases the amount of fuel used by the vehicle and undesirably increases the amount of noise and vibration which is communicated into the vehicle. It is therefore desirable to employ a preemptive slip operating strategy only when it is relatively likely that slip may occur (e.g., only when the vehicle is being driven upon a surface having a relatively low coefficient of friction).